In view of many disadvantages associated with x-ray image intensifiers and film of conventional types such as large bulk, complexity and incorporation of moving parts, U.S. Pat. No. 4,672,454 disclosed a flat amorphous silicon imaging panel, comprising an array of light-sensitive elements, as small as about 90 microns to a side and formed from an deposited semiconductor material such as amorphous silicon. For using such an x-ray imager effectively, however, appropriate means are required for providing signals representing the x-ray dose being received. For single shot fluorography (wherein snapshots are taken with an electronic device), a signal representing total integrated dose will be required. For fluoroscopy (wherein an electronic device is used for continuous imaging), on the other hand, a signal representing the instantaneous x-ray flux will be needed. Although various apparatus for exposure control have been available for radiography (or direct imaging on a film) and fluoroscopy with x-ray intensifiers and television cameras, there have not been any suitable exposure control apparatus or method for use with a flat amorphous silicon imaging panel.
U.S. Pat. No. 3,995,161, for example, disclosed an x-ray exposure device using a multiple-section ion chamber with integrating capacitors to provide measures of dose in several areas of a film, but ion chambers are incompatible with amorphous silicon panels because they are too bulky and require high voltages which are likely to interfere with the panel operation. Moreover, they require power supplies which produce noise that is likely to harm the signal quality from the panel. U.S. Pat. No. 4,517,594 disclosed an x-ray installation whereby a small percentage of light outputted from an x-ray image intensifier is re-imaged on a segmented photodetector, but there is no x-ray intensifier with a flat panel, nor is there any means for re-imaging. U.S. Pat. No. 4,171,484 disclosed a direct view fluoroscopic imaging system with an image intensifier tube and a high-voltage bias supply therefor. Dose signals are derived from the variations in the output from a phosphor display screen. This scheme, however, admits no selection of image sampling area, and amorphous silicon panels have no equivalent power supply means. U.S. Pat. No. 4,679,217 disclosed a film cassette with small scintillating screens for producing light to be detected by photodetectors in the cassette holder. This scheme requires auxiliary equipment to produce electrical signals for use by a generator and therefore prevents utilization of the cassette exposure control in any film holder except those designed with the auxiliary electronics included. Moreover, the light output of the screens, if used with amorphous silicon panels, does not truly represent the panel exposure as it would when used with a film. U.S. Pat. No. 4,442,537 disclosed a system which uses a television camera to measure the output from an x-ray image intensifier. The output from the television camera tube is used to generate a regulating signal for the control unit. If such a video signal is generated from an amorphous panel, it will not be produced until scanning occurs, and it will be too late to control the x-ray dose for fluorographic use.
As illustrated by this limited number of examples given above, prior art exposure control apparatus cannot fulfil all requirements for size, power consumption and compatibility with the characteristics of amorphous silicon panels.